1. Field of the Invention
The present invention relates to a digital to analog converter (DAC).
2. Description of Related Art
Most physical phenomena may be expressed as an analog value (or a varying analog signal) within a defined range of values. Temperature, pressure, voltage, current, light intensity, voice frequency, position, velocity, angular velocity, flow rate, etc., may all be expressed an analog signal having an instantaneous value at a point in time over a continuous range of possible values. Many analog ranges or scales may be used to accurately indicate the nature of the detected or measured phenomenon of interest.
Unfortunately, analog signals can not be manipulated by digital computational systems. Such systems require conversion of an analog signal into corresponding digital data. This process is referred to as analog to digital conversion (ADC). Analog signals may be converted into corresponding digital data by assigning digital data values to respective portions of an analog scale, and then comparing an instantaneous value for a received analog signal to the defined set of digital values.
Since digital computational systems output digital data, application of the data to an analog system requires a process of converting digital data into analog data (or an analog signal). Digital to analog conversion (D/A) is a process characterized by receiving a digital value represented by a digital code and then converting it into a predetermined analog value proportion to the digital value. During D/A conversion, a reference voltage (Vref) is commonly used to determine a maximum or maximum signal value for a constituent DAC. For example, 16 unique binary values may be represented by 4 bits of information. Thus, a voltage output (Vout) by a DAC receiving 4 bit digital data may have up to 16 different output levels. The actual analog output voltage Vout is proportional to the digital input value, and may be expressed as a multiple of the input digital value.
Given a fixed and constant reference voltage Vref, the output voltage Vout provided by a conventional DAC under the forgoing assumptions will only vary across a range of 16 possible voltage levels. This being the case, the output voltage Vout is far from being a true analog signal (i.e., a truly continuous signal). However, as number of possible output values is increased by increasing the number of bits of input data, the analog quality of the voltage output signal is improved.
One approach to the implementation of many competent DACs is the use of a sample and hold circuit implemented using an operational amplifier (Op-Amp). While offering significant design and performance benefits, this approach suffers from the presence of a parasitic capacitance at the input terminal of the Op-Amp. This parasitic capacitance has an adverse effect on the output of the DAC when modulating a voltage level of a non-inverting input terminal of the Op-Amp.